The present invention relates to a resonator constituting a radio frequency filter and the like, used for a radio frequency circuit device of a mobile communication system and the like.
Conventionally, a radio frequency communication system indispensably requires a radio frequency circuit element basically constructed of a resonator, such as a radio frequency filter. As a resonator for a low-loss radio frequency filter, often used is a dielectric resonator including a dielectric secured in a conductor shield.
FIGS. 19A and 19B are a perspective view and a cross-sectional view, respectively, of a conventional dielectric resonator 503 often used for a low-loss dielectric filter, which operates in a TE01δ mode as the base mode. The dielectric resonator 503 includes a cylindrical dielectric 501 and a cylindrical case 502 surrounding the dielectric 501 with a space therebetween. The dielectric 501 is mounted on a support and connected to the bottom portion of the case 502 via the support. The ceiling of the case 502 is apart from the top surface of the dielectric 501 by a given distance, and the sidewall (cylindrical portion) of the case 502 is apart from the cylindrical face of the dielectric 501 by a given distance.
Note that the case 502 is actually constructed of a case body and a lid as shown in FIG. 20 although it is shown in a simplified form in FIGS. 19A and 19B.
The above resonator using a TE mode (hereinafter, referred to as a “TE-mode resonator”) is superior to resonators using other modes in that it is small in loss and exhibits a good Q value, but has a disadvantage of being large in volume. Therefore, when a small resonator is desired, a resonator using a mode other than the TE mode as the base mode is used in some cases at the expense of the Q value characteristic to some extent.
FIG. 20 is a cross-sectional view of a radio frequency filter 530 having a resonator using a TM mode (hereinafter, referred to as a “TM-mode resonator”) that is considered a promising candidate for downsizing implementation. The resonator shown in FIG. 20 uses a TM mode called a TM010 mode among the other TM modes.
Referring to FIG. 20, the radio frequency filter 530 includes a cylindrical dielectric 540 and a case 531 composed of a case body 532 for housing the dielectric 540 and a lid 533. The case body 532 and the lid 533 are tightened together with bolts 535 so that the bottom surface of the lid 533 is in contact with the top face of the sidewall of the case body 532. The bottom surface of the lid 533 and the top surface of the bottom portion of the case body 532 are in contact with the top and bottom surfaces of the dielectric 540, respectively. In other words, the dielectric 540 is sandwiched between the lid 533 and the case body 532. The sidewall (cylindrical portion) of the case body 532 concentrically surrounds the dielectric 540 with a space therebetween. An input coupling probe 536 for input coupling with the dielectric 540 and an output coupling probe 537 for output coupling with the dielectric 540 are formed at the bottom portion of the case body 532.
However, it was found that the TM010 mode resonator shown in FIG. 20 failed to provide expected filter characteristics when it was actually prototyped. The present inventors consider the reason for this failure is as follows.
In the TE mode (TE01δ mode) resonator shown in FIGS. 19A and 19B, most of electromagnetic energy is confined within the dielectric, and only a small amount of radio frequency current flows to the side portion of the case 502. However, in the TM mode resonator shown in FIG. 20, a radio frequency 20 induced current flows in the side portion of the case body 532 in a direction parallel to the axial direction. Therefore, conductor loss comparatively largely influences the TM mode resonator. In particular, a large current flows across the corner at which the sidewall of the case body 532 and the lid 533 meet forming a connection Rcnct. If contact failure occurs at the connection Rcnct during the actual assembly of the resonator 530, this will presumably cause large deterioration in Q value and instability of operation. In addition, it has been found that if a gap exists between the top or bottom surface of the dielectric 540 and the lid 533 or the case body 532 due to size errors of components during the manufacture and the like, the resonant frequency sharply increases, and this possibly causes instability of operation. In particular, in the case of assembling a plurality of resonators to construct a filter, it is required to accurately fix the resonant frequency of the plurality of resonators. Therefore, in order to obtain desired filter characteristics while being free from instability of operation, considerably complicated work is presumably required.
In construction of a radio frequency filter using either type of resonator, the TE mode resonator or the TM mode resonator, the following three functions are important: that is,
(1) securing intense input/output coupling having a desired fractional bandwidth;
(2) having a resonant frequency adjusting mechanism that can reduce deterioration in the Q value of the resonator and also easily secure a wide frequency adjustable range; and
(3) having an inter-stage coupling degree adjusting mechanism that can easily secure a wide coupling degree adjustable range in the case of constructing a multi-stage radio frequency filter having a plurality of resonators. It is desired to implement a radio frequency filter having these functions.